1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device which contains a liquid crystal material layer wherein liquid crystals are dispersed in a matrix comprising a polymeric material, and which is substantially free from "cross-talk" and can exhibit a high brightness.
2. Description of the Related Art
It is known that liquid crystal display (LCD) devices can be directly operated by using an IC device under a low drive voltage at a low power consumption, and can be constructed as a compact and thin device. In particular, a TN type LCD can be operated under a low voltage at a low power consumption, and thus is widely utilized in various fields; for example, watches and desk-type electronic calculators.
The recent growing popularity of word processors, personal computers and other data processing devices has led to a demand for portable, small and thin devices, and to this end, the LCD devices are replacing the cathode ray tube (CRT) devices as the display element in those machines. With the LCD device, it is possible to reproduce the image of Chinese characters on the screens of those machines, since a great number of picture elements can be used in the LCD device in comparison with the number of picture elements used in the display device used for watches and desk calculators. Also, the LCD element is operated by a matrix display drive in which picture element electrodes are connected to signal lines in an X-Y matrix form. In this type of LCD device, the picture element electrodes corresponding to the picture elements are not independent from each other, and thus when a predetermined voltage is applied to one picture element electrode, the voltage is also applied to the adjacent picture element electrodes, whereby they are partly operated, i.e., an undesirable "cross-talk" occurs between one picture element electrode and adjacent picture element electrodes.
To eliminate this cross-talk, it is known to use a non-linear element consisting of a diode, for example, a metal-insulator-metal diode (MIM), or thin film transistor (TFT) for each picture element electrode, nevertheless it is very difficult to provide and arrange a large number (for example, several thousands to several hundreds of thousands) of diodes or thin film transistors corresponding to a large number of picture elements, all of which must have uniform properties and be free from defects, and thus there is an urgent need for the development of non-linear elements which can be easily provided with a uniform quality and will allow the use of an LCD device with a large display area.
In connection with the above, a new type of liquid crystal material usable for a display in a large display area has been developed through a new technology involving a polymer dispersed type liquid crystal material. With this technology, it is now possible to easily control the thickness of the liquid crystal material layer in the display device, and therefore, the polymer-dispersed type liquid crystal material can be formed as a layer having a large surface area, exhibits a very quick response, and allows the resultant display device to be given a wide angle of view without the use of a polarizer plate, and thus can be advantageously utilized to provide an LCD device having a large display surface area.
Nevertheless, the polymer-dispersed type liquid crystal material is disadvantageous in that this liquid crystal material requires a drive voltage of several tens to several hundreds of volts, which is remarkably higher than that of the TN type liquid crystal material, e.g., 5 volts or less, and therefore, a non-linear element having a higher voltage resistance than that of conventional non-linear elements, for example, thin film elements, must be used for the polymer-dispersed type LCD device.
The inventors of the present invention attempted to use a layer consisting essentially of varistor particles as a non-linear element, which is the liquid crystal display device in which the above-mentioned varistor layer is used can display clear images without the occurrence of cross-talk, even where the polymer-dispersed liquid crystal material needing a high drive voltage is used.
Nevertheless, the above-mentioned display device is disadvantageous in that, when an outside surface of a varistor layer formed on a signal line and a picture element electrode is close to a scanning electrode, a current sometimes flows from the signal line to the scanning electrode through the varistor layer, or from the picture element electrode to the scanning electrode through the varistor layer, so that a voltage between the picture element electrode and the scanning electrode is reduced, and the brightness of the display device is reduced. The above-mentioned disadvantages become significant with a decrease in the thickness of the liquid crystal material layer.
Usually, the varistor layer forms a convexity on a base plate in the liquid crystal display device, and this results in a relatively large thickness of the resultant liquid crystal display device. Also, the properties of the varistor layer may be deteriorated by solvent or additives contained in the liquid crystal material layer formed on the varistor layer.
In general, preferably the liquid crystal display device can be operated under a low drive voltage, and accordingly, the liquid crystal material layer has as small a thickness as possible. When the thickness of the liquid crystal material layer is reduced, however, the outside face of the varistor layer in the form of a convexity on the base plate comes into contact with an electrode formed on the opposite base plate, and if the thickness of the varistor layer is reduced to avoid the above-mentioned disadvantage, the deviation in the varistor voltage is increased.
Accordingly, desirably a liquid crystal display device having a stable varistor voltage and able to be operated under a low drive voltage is provided.
Furthermore, when each picture element electrode has a small size, and therefore the width of the varistor layer through which a signal line is connected to the picture element electrode is small, a threshold value voltage (Vth) of the varistor tends to fluctuate, and therefore, the contrast of each picture element becomes uneven.